cyst-

cyst

[sĭst]

An abnormal membranous sac in the body, containing a gaseous, liquid, or semisolid substance.

A small, capsulelike form of certain organisms that develops in response to adverse or extreme conditions. Under adverse conditions, for instance, dinoflagellates form nonmotile resting cysts that fall to the ocean or lake bottom and can remain there for years before reviving. Certain invertebrates, such as the water bear (phylum Tardigrada), also develop cysts.